The invention is based on a valve control unit for a fuel injection valve, in particular for a common rail injector.
A valve control unit of this kind for a fuel injection valve is known for instance from European Patent Disclosure EP 0 661 442 A1.
In the known valve control unit, there are two valve control chambers communicating continuously with one another in the housing body. The first valve control chamber communicates with an inflow conduit for fuel, which is connected to a high-pressure reservoir (common rail). The second valve control chamber has a passage to an outlet conduit, which can be opened and closed via a magnet valve. When the valve control unit is triggered, the outlet conduit is opened. As a result, the pressure in the second valve control chamber and thus in the first valve control chamber as well drops, so that the hydraulic imposition of pressure on one end of the valve control piston is also reduced. The other end of the valve control piston is connected to a nozzle needle for performing the injection. As soon as the hydraulic pressure imposition drops below the pressure imposition of the nozzle needle, the nozzle needle opens, so that the fuel can emerge through the injection opening into a combustion chamber. Manipulating the pressure ratios of the valve control chambers is done so as to control the valve control piston.
The terminal member of the valve control piston can be displaced in the injection event inside the first valve control chamber as far as a hydraulic stop (fuel cushion), which forms in the passage region between the first and second valve control chambers. This hydraulic stop is determined essentially by the size of the volume of the first valve control chamber. The known valve control unit has a first valve control chamber with a small volume, since only a small volume of the first valve control chamber assures that the hydraulic stop will not cause any vibration of the valve control piston and will have adequate rigidity. Because of the volumetric proportions of the first and second valve control chamber to one another, however, a considerable pressure gradient develops between the first and second valve control chambers. In the preinjection, the valve control piston can consequently move with a long valve stroke, so that a larger quantity of fuel is injected into the combustion chamber in the preinjection.
It would also be desirable to reduce the outer diameter of the terminal member of the valve control piston, so that this terminal member would have a lesser positive displacement cross section for fuel from the first valve control chamber. Reducing the outer diameter, however, necessarily increases the free volume of the valve control chamber, so that designing first valve control chamber in this way in turn leads to increased vibration on the part of the valve control piston at the hydraulic stop. Disadvantageously, the outer diameter of the terminal member of the valve control piston can therefore not be reduced, even though such a reduction, because of its reduced positive displacement cross section, would make an increased speed of motion of the terminal member possible.